Process for the production of noble metal thermoelectric wires



United States Patent Ufitice 3,34%,45? Patented Get. 31, 1967 3,349,467 PROCESS FOR THE PRODUCTION OF NOBLE METAL THERMOELECTRIC WIRES Walter Obrowski and Dieter Liebich, Hanan am Main,

Germany, assignors to Deutsche Golrlund Silber-Scheideanstalt vormals Roessler, Frankfurt am Main, Germany No Drawing. Filed Feb. 12, 1965, Ser. No. 432,405 13, 1964,

Claims priority, application Germany, Feb.

D 43,610 6 Claims. (Cl. 29-4205) ABSTRACT OF THE DISCLOSURE The present invention relates to a process for the production of noble metal, especially, platinum, thermoelectric wires of improved properties. Noble metal thermocouples are used in large numbers for high temperature measurement. In most of the thermocouples used the positive leg is formed of an alloy of platinum with 10% of rhodium and the negative leg is formed of pure platinum. The main field of use is for temperature measurements over 1100 C. to over 1600 C., preferably, between 1300 C. and 1500 C. Under these conditions of use the wires of the thermocouples are sub ect to various influences which may change their mechanical and electric properties from their original values. For example, the thermoelectric potential of the thermocouple may be changed by the take up of impurities, for instance, from the surrounding atmosphere and/or the surrounding ceramic parts because of the indirect action of the furnace atmospheres, especially, reducing furnace atmospheres, and therefore cause errors in the temperature indicated. Furthermore, materials may be taken up which form brittle intermetallic compounds with platinum engendering wire breakage or which form low melting eutectics with the platinum leading to fusion of the wires. In addition, the noble metal thermoelectric wires and especially the negative leg of very pure platinum in use at the high temperatures indicated are subjected to temperatures which are substantially higher than their recrystallization temperature and therefore undergo considerable grain growth. This grain growth which is observed in all pure metals leads to considerable reduction in mechanical strength and in elongation on break in comparison with the wires in their as produced state in which they are fine grained. This reduction in the mechanical properties during use of the thermoelectric wires renders them more susceptible to damage. Eventually, only strong mechanical shocks or vibrations sufiice to cause breakage of the coarse crystalline platinum thermoelectric wires.

It is an object of the present invention to provide a process which renders it possible to produce extremely fine grained, almost structureless platinum thermoelectric wires which have high hot strength and which do not suffer coarse grain formation even after being subjected to high temperatures for long periods of time and which retain their original mechanical properties even after long periods of use.

There has been no lack of attempts to overcome the normal disadvantages of, for example, pure platinum, even though high production costs were of no object. For example, it has been proposed that the grain growth transverse to the axis of the wire be decreased by enclosing a bundle of wires or rods in a tube and then processing such package by hot forging, rolling and wire drawing to a wire which in its longitudinal direction possesses a stretched structure which is supposed to provide high hot strength which only decreases slowly over extended periods of time. It is also known that sintered materials possess high strength and that the recrystallization temperature of such sintered materials is higher than that of the corresponding fused materials. As a consequence it has already been proposed to produce noble metal alloys by sintering methods employing as low sintering temperatures as possible and then processing the resulting sintered products by hot forging and cold working to wires which exhibit a fibrous structure such as was previously known in connection with the processing of extremely high melting metals, for instance, tungsten. A sintering temperature which is about half of the melting temperature has been found most advantageous. When higher sintering temperatures are used usable products are still obtained but they rapidly lose their advantageous properties after short periods of use at high temperatures. The highest sintering temperature should be 500 C. below the melting temperature. It also is known that easily oxidazable substances can be incorporated in noble metals or noble metal alloys during the smelting thereof and that such substances can be converted to their oxides after production of the end product by internal oxidation. On the other hand, it is also possible to add nonmetallic substances, especially oxides such as thorium oxide, directly during sintering. These additions are supposed to provide a supporting action in the products and therefore to increase their hot strength and also to retard recrystallization and therefore the feared coarse grain formation.

Of the above mentioned prior processes only the first has led to wires which could be used as thermocouple wires. The various sintering processes described either led to wires which were not suited for long use at high temperatures and which at high temperatures lose the good properties which they possess at low temperatures or their thermoelectric potential did not meet the requirements as to exactness and consistency over periods of time unless used in oxidizing atmospheres. Oxidizing atmospheres are required in view of their oxide content as, otherwise, there is the danger that the oxides might be reduced and thereby alter the electrical and/or mechanical properties to such an extent that the wires are entirely unsuitable for use in thermocouples at high temperatures.

It has now unexpectedly been found that the disadvantages of the above processes can be overcome so as to provide a product which fulfill all of the requirements for a noble metal thermoelectric wire in an almost ideal manner and therefore possesses equally good hot strength and elongation in all annealing atmospheres whether reducing or oxidizing even after hundreds of hours of annealing at very high temperatures of, for example, 1450 C., and simultaneously possesses an almost structureless extremely fine grained crystal structure, which even at temperatures slightly below, for example, C. below the melting point does not have any detectable tendency to recrystallize to produce coarse grains, combined with a high resistance against penetration of foreign Q substances from the surrounding medium because of the strongly branched structure.

The process according to the invention depends upon the use of an extremely finely divided noble metal, prefrably physically pure platinum powder of a grain size of less than 10,000 mesh per cm. and a highly fissured surface as is obtained by the reduction of noble metals at low temperatures, for example, of platinum ammonium chloride with hydrogen at temperatures between 200 and 500 C., preferably, below 350 C. or at about 300 C., followed by boiling out in acidified water to remove all ammonium chloride residues and to increase the surface area of the particles and drying at low temperatures. The reduction also can be carried out in a liquid medium with other known chemical reducing agents or it can be carried out electrochemically. In addition, gas phase decompositions or reductions of, for example, platinum carbonyl chloride at as low temperatures as possible produce powders suited for the production of thermoelectric wires according to the invention. Such powders are deep black in color and have a bulk factor of, at most 35 Compacts are produced from these powders in a known manner, taking care to maintain extreme purity as every i-mpurity leads to a change in the thermoelectric properties and therefore would make the charge unusable. The compacts with a pressed density of about 65% are then sintered in ceramic vessels, preferably, of very pure oxides such as, for example, aluminum oxide, at temperatures corresponding to the temperatures of intended use, preferably, at 1300 to 1500 C. for 2 to 8 hours, advantageously about 6 hours. Sinter bodies are thus obtained with about 98% of the theoretical density which can be cold worked without intermediate anneals to thermoelectric wires of a diameter, for example, 0.5 mm., for instance, by forging and/ or rolling followed by wire drawing. The thermoelectric wires are then given a stress relieving anmeal for to 20 minutes, preferably, for about 10 minutes at 1300 to 1500 C., preferably, 1450 C. to remove all mechanical disturbances which could change the thermoelectric potential in a non-desirable manner.

The platinum thermoelectric wires produced according to the invention after their production have a tensile strength of kg./mm. at room temperature and an elongation on break of about and a practically structureless texture, the grain size of which cannot be determined even upon 1000 enlargement. Platinum thermoelectric wires produced via smelting procedures have a tensile strength of 15 kg./mm. an elongation on break of and a grain texture of 700 grains per mm. After heating for 400 hours in uncontaminated air at 1450 C. which is the lowest period of utility expected of a PtRh-Pt thermocouple, the tensile strength at room temperature of the sintered product according to the invention is practically unchanged at 14 kg./mm. and its elongation on break is still 28%. The texture had suffered no discernable change during such heating. In contrast thereto, in platinum thermoelectric wires produced via smelting procedures such heating causes a drop in tensile strength to 7 kg./mm. and in elongation on break to 7% while simultaneously the grain structure is coarsened considerably so that only 1 to 3 grains per mm. are still present. These changes in the properties of the platinum produced via smelting procedures lead to the premature destruction of PtRh-Pt thermocouples which has been described above whether by the influence of mechanical stresses or of foreign substances.

We claim:

1. A process for the production of platinum metal thermoelectric wires having high hot strength and elongation and an almost structureless texture which is maintained even after heating for 400 hours at temperatures between 1300 and 1450 C. which comprises forming the platinum metal as a finely divided powder having a grain size below 10,000 mesh per cm. directly from a platinum metal compound at a temperature below 500 C., compressing such powder to form a compact, sintering such compact at a temperature between 1300 and 1500 C., cold working the sintered compact without an intermediate anneal to wire and subjecting such wire to a stress relieving anneal at a temperature between 1300 and 1500 C.

2. The process of claim 1 in which said powder is formed at a temperature between 200 and 350 C. by reduction of a platinum salt with hydrogen.

3. The process of claim 2 in which the platinum powder is formed by reduction of platinum ammonium chloride with hydrogen.

4. The process of claim 3 in which the platinum powder is boiled out with acidified water and dried before being compressed into compacts.

5. The process of claim 4 in which the reduction is carried out at about 300 C., the compact is sintered for about 6 hours at about 1400 C. and the stress relieving anneal is carried out at about 1450 C. for about 10 minutes.

6. The process of claim 1 in which said powder is formed by gas phase decomposition of platinum carbonyl chloride.

References Cited UNITED STATES PATENTS 7/1949 Middleton 213 1/1965 Gainsbury 75211 OTHER REFERENCES Goetzel, Treatise on Powder Metallurgy, vol. I, 1949, pp. 2, 3, 23-25, 60, 61. 

1. A PROCESS FOR THE PRODUCTION OF PLATINUM METAL THERMOELECTRIC WIRES HAVING HIGH FOR STRENGTH AND ELONGATION AND AN ALMOST STRUCTURELESS TEXTURE WHICH IS MAINTAINED EVEN AFTER HEATING FOR 400 HOURS AT TEMPERATURES BETWEEN 1300 AND 1450*C. WHICH COMPRISES FORMING THE PLATINUM METAL AS A FINELY DIVIDED POWDER HAVING A GRAIN SIZE BELOW 10,000 MESH PER CM.2 DIRECTLY FROM A PLATINUM METAL COMPOUND AT A TEMPERATURE BELOW 500*C., COMPRESSING SUCH POWDER TO FORM A COMPACT, SINTERING SUCH COMPACT AT A TEMPERATURE BETWEEN 1300 AND 1500* C., COLD WORKING THE SINTERED COMPACT WITHOUT AN INTERMEDIATE ANNEAL TO WIRE AND SUBJECTING SUCH WIRE TO A STRESS RELIEVING ANNEAL AT A TEMPERATURE BETWEEN 1300 AND 1500*C. 